1. Field of the Invention
The present invention relates to a pilot-operated directional control valve capable of changing between the internal and external pilot methods of introducing a pilot fluid.
2. Description of the Related
The pilot-operated directional control valve is a type of directional control valve used for switching the flow of a pressurized fluid such as compressed air. A pilot-operated directional control valve is composed of a main valve and a pilot valve, and switches a valve disc in the main valve by allowing a pilot fluid supplied from the pilot valve to act on the valve disc.
Such pilot-operated directional control valves are classified into internal and external pilot types depending on the method used to supply pilot fluid. Of the two, internal pilot-operated directional control valves guide part of a main fluid from a supply port in the main valve to the pilot valve; this fluid functions as the pilot fluid. External pilot-operated directional control valves, on the other hand, guide an exclusive pilot fluid (i.e., one distinct from the main fluid) to the pilot valve through an external pipe connected an external pilot port. The type of valve used will depend on the working conditions, so changes in working conditions often require changes between the internal and external pilot types.
To deal with this situation, directional control valves have been proposed that include a directional control mechanism for switching between the internal pilot type and the external pilot type.
As typically described in Japanese Utility Model Laid Open No. 64-17078, the directional control mechanism is composed of a plurality of parts and is integrated into a mounting hole of a complex shape formed in a valve body. As a result, such conventional directional control valves have a complex overall structure and require substantial time and labor for manufacturing or assembly. They are also costly. In addition, the individual parts, the mounting hole, and/or the channel opened in the mounting hole of these valves are all relatively cumbersome to process.